Lithium secondary batteries are widely used as main power sources or memory backup power sources of various electronic devices. In particular, in recent years, demand for lithium secondary batteries has been increasing as the use of small portable devices such as cellular phones and digital still cameras has been increasing.
While devices have been made small and lightweight, they are required to have higher functions, and memory capacity of the devices tends to increase. Therefore, both the main power sources and the backup power sources are required to be small and have high capacity. In recent years, studies have been made to use materials having high energy density as active materials for battery reaction.
In particular, silicon (Si) can be alloyed with lithium to composition of Li4.4Si, and it has a theoretical capacity as large as 4,199 mAh/g. Therefore, by using silicon as the negative electrode active material, a battery having a high capacity can be obtained.
However, a negative electrode including a negative electrode active material having a high energy density as Si swells and shrinks significantly while it absorbs and desorbs lithium and tends to collapse. Also, the more the amount of lithium contributing to the reaction is, the more the charge and discharge cycle characteristics lower. Therefore, various propositions have been made to suppress deterioration of the active material and to obtain good charge and discharge cycle characteristics.
For example, Patent Literature 1 discloses to combine a positive electrode using a lithium-containing manganese oxide and a negative electrode using a negative electrode active material including an Si phase alloyed with lithium and an alloy phase of Si and a transition metal element. Patent Literature 1 proposes to reduce the change of shapes of the electrodes by controlling the range of x in LixSi to 0≦x≦2.33 during the charge and discharge, thereby to improve the charge and discharge cycle characteristics.
Meanwhile, coin-type lithium secondary batteries are often used as backup power sources. In such uses, in many cases, a voltage to some extent is applied to the batteries at all times and the batteries are maintained in a charged state. In general, if a charged state is maintained for a long time, the capacity retention rate of the batteries lowers significantly. However, in the use of backup power sources, it is necessary to ensure sufficiently the capacity retention rate (continuous charge characteristics) even after being subjected continuously to the charged state.
Thus, in the coin-type lithium secondary batteries, it is necessary to have high charge and discharge cycle characteristic together with high continuous charge characteristics, and it is necessary to carry out optimal battery design on the whole.